When an exothermic polymerization reaction is conducted using a fluidized bed reactor, the reaction rate is to be restricted so as to have time enough for removing the released heat from the fluidized bed. A method to raise the removal rate of the reaction heat is to compress and cool the gas that is in a monomer state and is to be recycled back to the reactor, so that it is partially condensed. The condensed liquid is entrained by the recycle monomer gas stream and returned to the reactor together with the gas stream. This operation is referred to as "condensing mode" operation method of polymerization reaction.
It has long been recognized that the temperature of the recycle gas stream could not be lower than the dew point of the recycle gas stream. The dew point is a temperature at which liquid condensate begins to form in the gas stream. It is believed that the introduction of liquid into a gas phase fluidized bed reactor would inevitably result in plugging of the recycle lines, the heat exchanger and the distribution plate below the fluidized bed; non-uniformity of the monomer concentration inside the fluidized bed; and accumulation of liquid at the bottom, which would affect the continuous operation, even result in a complete shut down of the reactor, and in an extreme case, result in a collapse of the bed and deposition of the solid particle into a solid mass.
U.S. Pat. Nos. 4,543,399 and 4,588,790 issued to Jenkins, III et al. disclose that the "condensing mode" operation will not cause plugging and other problems affecting the continuous operation, but on the contrary permits a marked increase in the space-time-yield. This mode of operation enables both a reduction in the recycle gas stream temperature and absorption of heat, because of the evaporation of the liquid. The combination of the two functions allows much higher space-time-yields in this mode than that in the non-condensing mode.
UCC, Chinese Patent Application No. 85106978 points out that when condensing operation is conducted, the dew point of the recycle gas is intentionally raised so that the heat removal is further enhanced. One method is to increase the concentration of the condensable gas in the recycle gas stream. For example, a condensable gas inert to the catalyst, reactant and polymerization reaction product may be introduced into the recycle gas stream, thereby the dew point of the recycle gas stream is raised. The said gas introduced into the recycle gas stream may be introduced together with the supplementary gas or introduced using any other means or at any other position of the system. Examples of the condensable gas are saturated hydrocarbons including butane, pentane or hexane etc. Chinese Patent Application No. 93105791,4 to Exxon Chemical Co., further provides examples of volatilizable liquid hydrocarbons in a broader range as the condensing agent,: those selected from saturated hydrocarbons with 2-8 carbon atoms, such as propane, n-butane, iso-butane, n-pentane, iso-pentane, neopentane, n-hexane, iso-hexane and other saturated C.sub.6 hydrocarbons, n-heptane, n-octane and other C.sub.7 and C.sub.8 alkanes or the mixture thereof, among which C.sub.5 and C.sub.6 are preferred. Iso-pentane or n-hexane is selected in the examples, the construction of iso-pentane being 10.5 mole %.
In the prior arts operating in the condensing mode, only linear or branched saturated alkanes are disclosed to be selected as inert condensing agents with iso-pentane or n-hexane being preferred. It has never been disclosed that cycloalkanes can be selected as inert condensing agents. However, linear or branched alkanes have some intrinsic disadvantages.
If iso-pentane is used as the condensing agent, an amount of over 8 vol % has to be introduced into the recycle gas stream to achieve a desired space-time-yield due to the relatively light molecular weight, low boiling point, and small potential vaporization heat, and therefore the loss is rather great in the practical application because the content of iso-pentane in the gas phase is high. Taking another example wherein n-hexane is used as the condensing agent, because of the relatively heavy molecular weight and high boiling point, vicious accidents such as sticking, agglomeration of the resin, etc., will occur if its content in the recycle gas exceeds 6 vol % and the content of the liquid phase exceeds 20 wt % after condensation, thus the production has to be stopped for a treatment. What's more, n-hexane is rather difficult to be removed from the resin in the down stream treatment.
Through many times of comparative experiments, the inventors have surprisingly found that selection of cyclopentane as the condensing agent is more suitable for the condensing operation production in the fluidized bed polymerization reactor. Cyclopentane has the following advantages over iso-pentane:
The molecular weight approximately equals that of iso-pentane, but the vaporization heat is 25% greater than that of iso-pentane, i.e., the space-time-yield may be increased by 25% for the same amount of condensing agents.
The boiling point of cyclopentane at the normal pressure is 46.degree. C., while the pressure is 2500-2560 kPa and the temperature is 36-52.degree. C. at the reactor inlet. If the concentrations of both condensing agents are 4 vol %, the dew point of the recycle stream is 47.09.degree. C. when iso-pentane is used; the dew point is 57.08.degree. C. when cyclopentane is used. It can thus be seen that cyclopentane enables the recycle stream to reach the dew point more easily under the same conditions. Therefore, cyclopentane is more suitable to the inlet operation state of the fluidized bed reactor.
Because the molecular structure of cyclopentane differs from that of iso-pentane, the total amount of other condensed .alpha.-olefins induced by cyclopentane is 260% of that induced by iso-pentane in the same reactor inlet state and with the same stream composition. So the total amount of liquid in the recycle stream is relatively great and the amount of the removed heat is greater.
To reach the same space-time-yield, the cyclopentane content required in the recycle stream is 20 vol % lower than that of iso-pentane and therefore, the consumption is low, and the down-stream treatment is easier.
Because of the relatively low content of cyclopentane in the recycle stream, the probability of agglomeration of the fluidized bed is greatly reduced under a super-condensed state and the control and regulation of the inlet components are also facilitated.
Cyclopentane has the following advantages over n-hexane:
The molecular weight is lighter than that of n-hexane. The power of the recycle compressor is 22% lower than that when n-hexane is used.
The normal pressure boiling point is lower than that of n-hexane and the difference between the dew point temperature under the inlet state of the reactor and the lowest operation temperature of the reactor is as great as 30.92.degree. C., so the probability of the agglomeration of the resin resulted by the accumulation of the liquid above the distribution plate is greatly reduced.
The potential vaporization heat is 15% greater than that of n-hexane and more heat is removed, and thus the space-time-yield increases by 15% for the same amount of condensing agent.
It is easier to remove in the down stream treatment system at the same space-time-yield, therefore the fixed investment of the degassing and recovery systems is saved and the consumption is low.
Under the same amount of the liquid, the monomers (such as ethylene, co-polymerization monomer) and hydrogen etc. dissolved in cycopentane are about 13% more than those dissolved in n-hexane. Therefore it is easier to atomize after throttle and the plugging probability of the distribution plate is greatly reduced.
The object of the present invention is, therefor, to select cyclopentane with a moderate molecular weight, a boiling point closer to the inlet temperature, a greater potential vaporization heat and more heat removal so as to overcome the intrinsic disadvantages of the presently used condensing agent in the fluidized bed reactor process operated in the condensing mode, thus greatly increasing the space-time yield of the polymer at the same amount of condensing agent. Meanwhile, the concentration of the other components in the recycle gas stream is increased due to the decrease of the amount of the condensing agent, so that the composition of the stream is easy to regulate and also the accidents of the resin sticking and agglomeration are not likely to occur; and the removal and recovery of the condensing agent are easy to be conducted in the subsequent treatment and thereby the energy consumption is reduced.